On synthesizers or any other electronic device for generating sound, it is commonplace to have one or more haptic controllers to allow the instrumentalist to control various characteristics of the generated sound, such as its volume, its vibrato, its dynamics, etc.
By way of example, the volume pedals of organs with electromechanical wheels are known, or else the volume pedal branded BOSS EV5 with a MIDI interface that can be linked to a synthesizer. In general, these pedals or haptic foot controllers are provided with a potentiometer, the output of which is digitized in order to generate a signal that is proportional to the depression of the pedal.
On instruments of ondes Martenot type, a key called the intensity key or expression key can be found that allows the volume of the sound generated by the instrument to be controlled. In a manner known per se, the key is mounted as a lever so that pressing the key causes a bag of conductive powder to be crushed that conveys the electrical signal carrying the sound. The crushing of the bag causes the resistivity of the powder to be modified and, consequently, the amplitude of the sound to be modified. The use of such a bag allows a relationship between the force applied to the key and the amplitude of the sound that is very close to the psycho-physiological perception of the musician.
However, such an achievement is tricky to implement, and it appears that the bag of powder is difficult to reproduce and complex to implement in practice. Moreover, this device is solely analog and is not suited to use as a haptic controller for modern instruments.